Acquisition of information adaptable for electronic processing applications directly from printed original documents has become greatly desirable as an adjunct to electronic document production. An important reason for its desirability is the subsequent capability of manipulation of the electronically stored information for editing, compiling and using the information in forms other than that in which it was originally available. Accordingly, it is desirable to have an image information input capability for such information. In addition to the production of new documents from electronically stored information, it will also be appreciated that some copying functions available in light lens-type copiers, copying images directly from original documents, may be more readily accomplished if image information is available electronically. Thus, with the capability of electronic input of information, coupled with available output devices, functions such as duplex copying, image rotation, cropping, editing, etc., are possible without the requirement of difficult mechanical manipulation of originals and copies.
In duplex scanning, the scan of one side of a typical two-sided piece of translucent paper will contain information from both sides of the page. The high contrast information of the front side will be combined with a low contrast version of the image from the back side of the page. This low contrast information from the back side of the page is called "show-through". In simplex scanning operations where, for example, multiple two-sided pages of a book are being scanned for storage during archiving applications or for duplication, the text of subsequent pages may also "show through" the original target page together with the back side of this original. Thus, layers of show through text or noise interfere with a clean transfer of information such as text from the first page being scanned.
One method for mechanically reducing show-through is to place a black page on the back side of the page while scanning. The light that goes through the page is absorbed by the black backing and, although there is a significant reduction of the show-through by this method, there is a small residual low contrast image of the back side remaining in the scanned image due to light scattering off the back side of the paper. Also, this process requires further user effort therefore becoming a burden to the user.
There are circumstances for which it is not possible to mechanically remove show-through. One is when both sides of the page are scanned simultaneously. In this case, both sides of the page are illuminated and imaged at the same time. Under these conditions, the information on each side will show through to the opposite side. Another circumstance is when a post-scanning correction for show-through is needed for an image scanned on a scanner with a white cover.
The problem of show-through has sometimes been addressed mechanically, as in U.S. Pat. No. 4,743,974 to Lockwood, which shows simultaneous duplex scanning of duplexed documents, with sensors that are slightly offset. The close placement of sensors for simultaneous duplex scanning is desirable for space consideration reasons.
With only the scanned image from one side, there is no way of distinguishing between the low contrast "show-through" information from the back side and the low contrast information from the front side of the page. Additional information is needed to be able to distinguish between the two types of low contrast information. References described herein are hereby incorporated by reference for their teachings.